Measurement of cardiac output in man with a nonrecirculating indicator

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The present investigation was undertaken to evaluate the utility of constant-rate injection of a nonrecirculating indicator (H2) for the measurement of cardiac output in man. 42 patients were studied during cardiac catheterization and 8 during acute complications of arteriosclerotic heart disease, including acute myocardial infarction. Pulmonary (or systemic) arterial H2 concentration was measured chromatographically from 2.0 ml blood samples drawn during constant-rate injection of dissolved H2 into the systemic venous circulation (or left heart). The chromatograph was a thermal conductivity unit housed in a constant-temperature water bath to achieve an improved signal-to-noise ratio. Intrapulmonary H2 elimination from mixed venous blood was measured directly in 14 patients and averaged 98 ±1.5% (SD). Reproducibility of output measurements was evaluated using triplicate determinations obtained over 45-60 sec in 25 consecutive patients. Coefficients of variation (SD/Mean × 100) averaged 3.4 ±2.0%, making it possible to evaluate relatively small changes in measured output with conventional statistical tests. Individual measurements could be repeated at 10-15 sec intervals. Comparisons of H2 and direct Fick measurements were made in 14 patients; H2 outputs averaged 106 ±4% (SEM) of Fick outputs (P > 0.1). Comparisons of H2 and dye dilution measurements were performed in an additional 24 patients. Seven had angiographically-negligible valvular regurgitation and dye outputs averaged 106 ±3% of H2 outputs (P > 0.1). 17 had moderate-to-severe regurigation and dye outputs averaged 91 ±4% of H2 outputs (P < 0.05), suggesting a small but systematic error due to undetected recirculation of dye. The H2 technique appears advantageous for rapidly repeated determinations of output, for quantitation of small changes in output, and for situations in which recirculation of conventional indicators is a potentially significant problem.

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